The time dimension of left/right is like stepping sideways across to neighboring parallel universes. The farther you move, the more reality diverges. The calendar date remains the same, but reality changes. Called parallel universes, quantum realities, or divergent worldlines. (Illustrated in TNG: "Parallels" and any Stargate SG1 episode dealing with the Quantum Mirror.)

The time dimension of up/down means venturing away from the plane of reality altogether into the realm of thought, dreams, and nightmares. The farther you travel up or down from Now/This, the farther you recede from actual reality into the Imaginary. Again, the calendar date stays the same, and your quantum universe doesn't change, but the Real becomes Imaginary. (e.g. TNG: "Where No One Has Gone Before", TNG: "Journey's End".)